Methane emissions from contrasting urban freshwaters: Rates, drivers, and a whole‐city footprint

Abstract

Global urbanization trends impose major alterations on surface waters. This includes impacts on ecosystem functioning that can involve feedbacks on climate through changes in rates of greenhouse gas emissions. The combination of high nutrient supply and shallow depth typical of urban freshwaters is particularly conducive to high rates of methane (CH4) production and emission, suggesting a potentially important role in the global CH4 cycle. However, there is a lack of comprehensive flux data from diverse urban water bodies, of information on the underlying drivers, and of estimates for whole cities. Based on measurements over four seasons in a total of 32 water bodies in the city of Berlin, Germany, we calculate the total CH4 emission from various types of surface waters of a large city in temperate climate at 2.6 ± 1.7 Gg CH4/year. The average total emission was 219 ± 490 mg CH4 m−2 day−1. Water chemical variables were surprisingly poor predictors of total CH4 emissions, and proxies of productivity and oxygen conditions had low explanatory power as well, suggesting a complex combination of factors governing CH4 fluxes from urban surface waters. However, small water bodies (area <1 ha) typically located in urban green spaces were identified as emission hotspots. These results help constrain assessments of CH4 emissions from freshwaters in the world's growing cities, facilitating extrapolation of urban emissions to large areas, including at the global scale.

Small artificial waterbodies are widespread and persistent emitters of methane and carbon dioxide

“…We assume that NH 4 is essentially a signal of agricultural/urban run‐off, which will deliver high inputs of N, P and labile organic matter into waterbodies thus fuelling CH 4 production. There were significantly higher CH 4 fluxes in waterbodies with the largest TP concentrations, in keeping with a wide body of literature showing higher emissions in nutrient‐rich waterbodies including ditches and artificial ponds (Audet et al, 2020; Beaulieu et al, 2019; Herrero Ortega et al, 2019; Ollivier et al, 2019a; Peacock et al, 2017, 2019; Webb, Hayes, et al, 2019). However, high TP concentrations did not guarantee high fluxes, and this is in agreement with a recent synthesis of ditch CH 4 emissions (Peacock et al, 2021).…”

“…There were significantly higher CH 4 fluxes in waterbodies with the largest TP concentrations, in keeping with a wide body of literature showing higher emissions in nutrient-rich waterbodies including ditches and artificial ponds (Audet et al, 2020;Beaulieu et al, 2019;Herrero Ortega et al, 2019;Ollivier et al, 2019a;Peacock et al, 2017Peacock et al, , 2019Webb, Hayes, et al, 2019). However, high TP concentrations did not guarantee high fluxes, and this is in agreement with a recent synthesis of ditch CH 4 emissions (Peacock et al, 2021 Note: Soil type had a significant effect on CO 2 concentrations (p < 0.001, d = 0.71, effect size = medium) and CH 4 concentrations (p = 0.041, d = 0.19, effect size = very small).…”

Temporal Patterns of Methane Emissions From Two Streams With Different Riparian Connectivity

“…CH 4 fluxes from both streams to vary between 0.08 and 7.02 mmol m −2 d −1 are within the literature range (Crawford, Lottig, et al, 2014;Stanley et al, 2016;Striegl et al, 2012). We confirmed that although streams are well-aerated, CH 4 oversaturation is ubiquitous in the DOC-rich boreal (Campeau & Giorgio, 2014;Natchimuthu et al, 2017;Rasilo et al, 2017) and temperate streams (Herrero Ortega et al, 2019;Hope et al, 2001;Jones & Mulholland, 1998b;Schrier-Uijl et al, 2011;Smith & Böhlke, 2019). Halbedel and Koschorreck (2013) estimated CO 2 emissions from both streams between 23 and 355 mmol m −2 d −1 with higher CO 2 emission rates in the Hassel stream (252 ± 53 mmol m −2 d −1 ).…”

Drainage Ditches Contribute Considerably to the CH4 Budget of a Drained and a Rewetted Temperate Fen

“…Other studies have found that water chemical properties are not always good predictors for CH 4 concentrations or uxes in aquatic systems (Ortega et al 2019). Apart from nutrient and substrate availability in the water column, the nutrient status of the sediment is important for methanogenesis.…”